Golf ball

ABSTRACT

Disclosed is a golf ball having a cover formed from a thermoplastic polyurethane material which can be recycled for molding, which exhibits high restitution, and which exhibits excellent scuff resistance and discoloration resistance. The cover is formed from a composition (C) containing, as predominant components, the following components (A) and (B): 
     (A) a polyurethane material mixture of a thermoplastic polyurethane material (a-1) and dicyclohexyl-4,4′-methane diisocyanate (a-2), and 
     (B) an isocyanate mixture in which an isocyanate compound (b-1) having at least two isocyanate groups serving as functional groups in the molecule is dispersed in a thermoplastic resin (b-2) which is substantially non-reactive with the isocyanate groups.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a golf ball having a cover formed froma thermoplastic polyurethane material; and more particularly to a golfball having a cover formed from a thermoplastic polyurethane materialwhich can be recycled for molding, which exhibits high restitution, andwhich exhibits excellent scuff resistance.

2. Description of the Related Art

In recent years, polyurethane materials have become of interest asmaterials for forming a golf ball cover. Polyurethane materials areclassified into thermosetting polyurethane materials and thermoplasticpolyurethane materials, and a process for forming a thermosettingpolyurethane material into a product differs from a process for forminga thermoplastic polyurethane material into a product. A thermosettingpolyurethane material can be formed into a product through the followingprocedure: a urethane prepolymer having an isocyanate end group and acuring agent such as polyol or polyamine, which serve as liquid rawmaterials, are mixed under heating; and the resultant mixture is feddirectly to a mold and then heated, to thereby allow urethane curingreaction to proceed.

Many studies have heretofore focused on golf balls formed fromthermosetting polyurethane materials. For example, U.S. Pat. Nos.5,334,673, 6,117,024, and 6,190,268 disclose such golf balls. Meanwhile,U.S. Pat. Nos. 5,006,297, 5,733,428, 5,888,437, 5,897,884, and 5,947,843disclose forming methods of thermosetting polyurethane materials.

Since a thermosetting polyurethane material exhibits nothermoplasticity, the material and a product formed from the materialcannot be recycled. In addition, when a thermosetting polyurethanematerial is employed for forming a specific product such as a golf ballcover (i.e., a product which covers a core), efficient production of theproduct is not attained, since the heating-curing step and the coolingstep of the material requires long time, and high reactivity andinstability of the material make control of the molding time verydifficult.

In the case where a thermoplastic polyurethane material is formed into amolded product, the product is not directly obtained through reaction ofraw materials, but is formed from a linear polyurethane material—anintermediate—which has been synthesized by employment of raw materialsand a synthesis method, the raw materials and the method differing fromthose employed in the case of the aforementioned thermosettingpolyurethane material. Such a linear polyurethane material exhibitsthermoplasticity, and is cured through cooling. Therefore, such apolyurethane material can be molded by use of an injection moldingmachine. Injection molding of a thermoplastic polyurethane material is atechnique suitable for forming a golf ball cover, since the molding timeof a thermoplastic polyurethane material is much shorter than that of athermosetting polyurethane material, and a thermoplastic polyurethanematerial is suitable for precise molding. Meanwhile, a thermoplasticpolyurethane material can be recycled, and is thus environmentallyfriendly. U.S. Pat. Nos. 3,395,109, 4,248,432, and 4,442,282 disclosegolf balls formed from thermoplastic polyurethane materials.

However, when a golf ball cover is formed from a conventionalthermoplastic polyurethane material, the resultant golf ball is notsatisfactory in terms of feeling on impact, controllability,restitution, and scuff resistance upon being hit with an iron.

In order to solve such a problem, Japanese Patent Application Laid-Open(kokai) No. 9-271538 discloses a golf ball cover formed from athermoplastic polyurethane material exhibiting high restitution.However, the disclosed golf ball cover is not satisfactory in terms ofscuff resistance upon being hit with an iron.

Japanese Patent Application Laid-Open (kokai) No. 11-178949 discloses agolf ball cover exhibiting relatively excellent scuff resistance uponbeing hit with an iron, which predominantly contains a reaction productformed from a thermoplastic polyurethane material and an isocyanatecompound. When the cover is formed, an isocyanate compound such as adiisocyanate or a block isocyanate dimer, serving as an additive, isadded to a thermoplastic polyurethane material in the course of heating,melting, and mixing by use of an extruder, or in the course of injectionmolding, to thereby allow reaction to proceed.

However, in the case of molding of the cover disclosed in JapanesePatent Application Laid-Open (kokai) No. 11-178949, since an isocyanatecompound must be handled with great care due to its inactivation bymoisture, obtaining a stable reaction product is difficult. Meanwhile, ablock isocyanate exhibiting moisture resistance is not suitable forforming the cover, since a blocking agent issues a strong odor when theisocyanate is thermally dissociated. When an isocyanate compound assumesthe form of powder or solution, control of the amount of the compoundwhich is added to a thermoplastic polyurethane material is difficult,and therefore cover properties cannot be controlled adequately. Inaddition, since the thermoplastic polyurethane material differs inmelting point and melt viscosity from the isocyanate compound, thoroughand satisfactory kneading thereof may fail to be attained in a moldingapparatus. Therefore, in the technique disclosed in the abovepublication, the effect of moisture on a cover material and the amountof an additive is not satisfactorily controlled, resulting in failure toproduce a golf ball cover which is satisfactory in terms of improvementof scuff resistance.

Japanese Patent Application Laid-Open (kokai) No. 11-178949 discloses analiphatic isocyanate-based thermoplastic polyurethane material to beused as a desirable thermoplastic polyurethane material. However, sincethe thermoplastic polyurethane material is highly reactive withisocyanate and its reaction is difficult to control, the polyurethanematerial involves the following problems: gelation easily occurs beforeinjection molding, and sufficient plasticity cannot be maintained;gelation may occur during molding of a cover; and the polyurethanematerial cannot be recycled, due to gelation. Because of such problems,the thermoplastic polyurethane material is difficult to use in practice.

Japanese Patent Publication (kokoku) No. 58-2063 (U.S. Pat. No.4,347,338) discloses a process for producing a thermosettingpolyurethane product, in which a compound having two or more isocyanategroups is mixed with a thermoplastic resin which is non-reactive with anisocyanate group, the resultant mixture is incorporated into athermoplastic polyurethane material, and the resultant material issubjected to molding by use of a molding machine. However, the purposeof the technique disclosed in the above publication is to improve thepolyurethane product only in terms of solvent resistance and durabilityagainst continuous, repeating friction, and the publication does notdisclose use of the aforementioned forming material as a material of agolf ball cover. There still exists demand for a golf ball covermaterial which can provide a golf ball with various necessaryproperties, such as restitution, total distance, spin performance,controllability, feeling on impact, scuff resistance, cut resistance,and discoloration resistance.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide a golf ball having a cover formed from a thermoplasticpolyurethane material which can be recycled for molding, which exhibitshigh restitution, and which exhibits excellent scuff resistance anddiscoloration resistance.

In order to achieve the above object, the present invention provides thefollowing golf balls.

(1) A golf ball comprising a core and a cover therefor, wherein thecover is formed from a composition (C) containing, as predominantcomponents, the following components (A) and (B):

(A) a polyurethane material mixture of a thermoplastic polyurethanematerial (a-1) and dicyclohexyl-4,4′-methane diisocyanate (a-2), and

(B) an isocyanate mixture in which an isocyanate compound (b-1) havingat least two isocyanate groups serving as functional groups in themolecule is dispersed in a thermoplastic resin (b-2) which issubstantially non-reactive with the isocyanate groups.

(2) A golf ball according to (1), wherein the thermoplastic polyurethanematerial (a-1) is synthesized from a polyether polyol and an aromaticdiisocyanate.

(3) A golf ball according to any one of (2), wherein the polyetherpolyol is a polytetramethylene glycol having an average molecular weightof at least 2,000.

(4) A golf ball according to (2) or (3), wherein the aromaticdiisocyanate is 4,4′-diphenylmethane diisocyanate.

(5) A golf ball according to any one of (1) through (4), wherein, in thepolyurethane material mixture (A), the ratio by weight of thethermoplastic polyurethane material (a-1) to dicyclohexyl-4,4′-methanediisocyanate (a-2) is 100:0.5 to 100:5.

(6) A golf ball according to any one of (1) through (5), wherein theisocyanate compound (b-1) in the isocyanate mixture (B) is4,4′-diphenylmethane diisocyanate.

(7) A golf ball according to any one of (1) through (6), wherein thethermoplastic resin (b-2) in the isocyanate mixture (B) is athermoplastic polyester resin.

(8) A golf ball according to any one of (1) through (7) wherein, in theisocyanate mixture (B), the ratio by weight of the thermoplastic resin(b-2) to the isocyanate compound (b-1) is 100:5 to 100:100.

(9) A golf ball according to any one of (1) through (8), wherein, in thecomposition (C), the ratio by weight of the thermoplastic polyurethanematerial (A) to the isocyanate mixture (B) is 100:1 to 100:40.

(10) A golf ball according to any one of (1) through (9), wherein thematerial of the cover has a surface hardness of 40 to 80 as measured byuse of a D-type durometer, and a restitution elastic modulus of at least45%.

(11) A golf ball according to any one of (1) through (10), wherein thematerial of the cover can be recycled for molding.

DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The present invention will next be described in more detail. Firstly,components (A) and (B) and composition (C) will be described.

(A) Polyurethane Material Mixture

(a-1) Thermoplastic Polyurethane Material

The thermoplastic polyurethane material includes soft segments formed ofa polymeric polyol (polymeric glycol), a chain extender constitutinghard segments, and a diisocyanate. No particular limitation is imposedon the polymeric polyol serving as a raw material, and the polymericpolyol may be any one selected from polymeric polyols which areconventionally employed in the technical field related to thermoplasticpolyurethane materials. Examples of the polymeric polyol includepolyester polyols and polyether polyols. Of these, polyether polyols aremore preferred to polyester polyols, since a thermoplastic polyurethanematerial having high restitution elastic modulus and exhibitingexcellent low-temperature properties can be synthesized. Examples of thepolyether polyols include polytetramethylene glycol and polypropyleneglycol. From the viewpoints of restitution elastic modulus andlow-temperature properties, polytetramethylene glycol is particularlypreferred. The average molecular weight of the polymeric polyol ispreferably 1,000 to 5,000. The average molecular weight is morepreferably 2,000 to 4,000, in order to synthesize a thermoplasticpolyurethane material having high restitution elastic modulus.

Any chain extender which is conventionally employed in the technicalfield related to thermoplastic polyurethane materials is preferablyused. Examples of the chain extender include, but are not limited to,1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol,1,6-hexanediol, and 2,2-dimethyl-1,3-propanediol. The average molecularweight of the chain extender is preferably 20 to 15,000.

Any diisocyanate which is conventionally employed in the technical fieldrelated to thermoplastic polyurethane materials is preferably used.Examples of the diisocyanate include, but are not limited to, aromaticdiisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluenediisocyanate, and 2,6-toluene diisocyanate; and aliphatic diisocyanatessuch as hexamethylene diisocyanate. Some diisocyanates involvedifficulty in controlling cross-linking reaction during injectionmolding. In the present invention, 4,4′-diphenylmethane diisocyanate,which is an aromatic diisocyanate, is most preferred, in considerationof stability in reaction with the below-described isocyanate mixture(B).

In the present invention, a thermoplastic polyurethane materialsynthesized from a polyether polyol and an aromatic diisocyanate is mostpreferred; the polyether polyol is polytetramethylene glycol having anaverage molecular weight of at least 2,000, and the aromaticdiisocyanate is 4,4′-diphenylmethane diisocyanate.

Preferred examples of the thermoplastic polyurethane material containingthe aforementioned materials include commercially available polyurethanematerials, such as Pandex T-8290, T-8295, and T-8260 (products of DICBayer Polymer Ltd.), and Resamine 2593 and 2597 (products ofDainichiseika Color & Chemicals Mfg. Co., Ltd.).

(a-2) Dicyclohexyl-4,4′-Methane Diisocyanate (Hydrogenated MDI)

The purpose of addition of dicyclohexyl-4,4′-methane diisocyanate (a-2)to the thermoplastic polyurethane material (a-1) is to impart across-linking structure to the thermoplastic polyurethane material, andto enhance the strength of the thermoplastic polyurethane material, tothereby reduce the amount of the isocyanate mixture (B) to be added tothe cover material. When the isocyanate mixture (B) is added to thecover material in a large amount, the cover material is considerablydiscolored. However, when dicyclohexyl-4,4′-methane diisocyanate (a-2)to the thermoplastic polyurethane material (a-1) in advance, if theamount of the isocyanate mixture (B) is reduced, the cover materialexhibits scuff resistance similar to the case where the amount of theisocyanate mixture (B) is not reduced, and discoloration of the covermaterial can be prevented.

In the polyurethane material mixture (A), the ratio by weight of thethermoplastic polyurethane material (a-1) to dicyclohexyl-4,4′-methanediisocyanate (a-2) is preferably 100:0.5 to 100:5, more preferably 100:1to 100:4. When the ratio of dicyclohexyl-4,4′-methane diisocyanate (a-2)to the thermoplastic polyurethane material (a-1) is excessively low,scuff resistance improving effect becomes unsatisfactory, whereas whenthe ratio of dicyclohexyl-4,4′-methane diisocyanate (a-2) to thethermoplastic polyurethane material (a-1) is excessively high, viscosityof the polyurethane material mixture (A) increases due to gelation,resulting in poor moldability.

(B) Isocyanate Mixture

The isocyanate mixture (B) is obtained by dispersing the isocyanatecompound (b-1) having at least two isocyanate groups serving asfunctional groups in the molecule in the thermoplastic resin (b-2) whichis substantially non-reactive with the isocyanate groups. Theaforementioned isocyanate compound (b-1) is preferably an isocyanatecompound which is conventionally employed in the technical field relatedto thermoplastic polyurethane materials. Examples of the isocyanatecompound include, but are not limited to, aromatic diisocyanates such as4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, and2,6-toluene diisocyanate; and aliphatic diisocyanates such ashexamethylene diisocyanate. Of these, 4,4′-diphenylmethane diisocyanateis most preferred, in consideration of reactivity and operationalsafety.

The aforementioned thermoplastic resin (b-2) is preferably a resinhaving low water-absorbability and high compatibility with thethermoplastic polyurethane material. Examples of the resin includepolystyrene resins, polyvinyl chloride resins, ABS resins, polycarbonateresins, and polyester elastomers (e.g., polyether-ester block copolymersand polyester-ester block copolymers). Of these, in consideration ofrestitution elasticity and strength, polyester elastomers, inter alia,polyether-ester block copolymers, are particularly preferred.

In the isocyanate mixture (B), the ratio by weight of the thermoplasticresin (b-2) to the isocyanate compound (b-1) is preferably 100:5 to100:100, more preferably 100:10 to 100:40. When the ratio of theisocyanate compound (b-1) to the thermoplastic resin (b-2) isexcessively low, a large amount of the isocyanate mixture (B) must beadded to the thermoplastic polyurethane material (A), in order toachieve a successful cross-linking reaction between the isocyanatecompound (b-1) and the thermoplastic polyurethane material (A). As aresult, the thermoplastic resin (b-2) greatly affects the thermoplasticpolyurethane material (A), resulting in unsatisfactory properties of thecomposition (C). In contrast, when the ratio of the isocyanate compound(b-1) to the thermoplastic resin (b-2) is excessively high, thorough andsatisfactory kneading of the isocyanate compound (b-1) into thethermoplastic resin (b-2) is not attained, and thus preparation of theisocyanate mixture (B) becomes difficult.

The isocyanate mixture (B) can be obtained through, for example, thefollowing procedure: the isocyanate compound (b-1) is incorporated intothe thermoplastic resin (b-2), and the resultant mixture is completelykneaded by use of a mixing roll or a banbury mixer at 130 to 250° C.,followed by pelletization or pulverization after cooling. Preferredexamples of the isocyanate mixture (B) include commercially availableisocyanate mixtures such as Crossnate EM30 (product of DainichiseikaColor & Chemicals Mfg. Co., Ltd.).

(C) Composition

The composition (C) comprises, as predominant components, a polyurethanematerial mixture (A) and a isocyanate mixture (B). In the composition(C), the ratio by weight of the thermoplastic polyurethane material (A)to the isocyanate mixture (B) is preferably 100:1 to 100:100, morepreferably 100:5 to 100:50, much more preferably 100:10 to 100:30. Whenthe ratio of the isocyanate mixture (B) to the thermoplasticpolyurethane material (A) is excessively low, the isocyanate mixture (B)exerts insufficient cross-linking effect, whereas when the ratio isexcessively high, unreacted isocyanate imparts a color to the resultantcomposition.

In the present invention, the cover-forming material may contain othercomponents in addition to the aforementioned components. Examples ofsuch “other components” include thermoplastic polymer materials otherthan the thermoplastic polyurethane material, such as polyesterelastomer, polyamide elastomer, ionomer resin, styrene block elastomer,polyethylene, and nylon resin. In this case, the incorporation amount ofthermoplastic polymer materials other than the thermoplasticpolyurethane material is 0 to 100 parts by weight, preferably 10 to 75parts by weight, more preferably 10 to 50 parts by weight, on the basisof 100 parts by weight of the thermoplastic polyurethane material whichserves as an essential component. The incorporation amount isappropriately determined in accordance with various purposes, includingregulation of the hardness of the cover-forming material and improvementof the restitution, fluidity, and adhesion of the cover-formingmaterial. If desired, the cover-forming material may further containvarious additives, such as pigments, dispersants, antioxidants,light-resistant stabilizers, UV absorbers, and release agents.

A cover of the golf ball of the present invention can be formed through,for example, the following procedure: isocyanate mixture (B) is added topolyurethane material mixture (A) and then dry-mixed, and a cover isformed from the resultant mixture around a core by use of an injectionmolding apparatus. The molding temperature varies with the type of thethermoplastic polyurethane material (a-1), but is typically 150 to 250°C.

In the resultant golf ball cover, reaction or cross-linking is thoughtto proceed as follows: an isocyanate group is reacted with a residual OHgroup of the thermoplastic polyurethane material, to thereby form aurethane bond; or an isocyanate group is added to a urethane group ofthe thermoplastic polyurethane material, to thereby form an allophanateor biuret cross-linking structure. In this case, although cross-linkingproceeds insufficiently immediately after injection molding of thecover-forming material, cross-linking proceeds through annealing afterinjection molding, and the resultant golf ball cover is endowed withuseful properties. As used herein, the term “annealing” refers to agingthrough heating at a certain temperature for a predetermined period oftime, or aging at room temperature for a predetermined period of time.

The surface hardness of the cover material of the golf ball of thepresent invention is preferably 40 to 80, more preferably 43 to 60, muchmore preferably 45 to 55, as measured by use of a D-type durometer inaccordance with JIS-K6253. When the surface hardness of thecover-forming material is excessively low, the resultant golf ball tendsto produce excessive back-spin upon being hit with an iron; i.e.,controllability of the golf ball is impaired. In contrast, when thesurface hardness of the cover-forming material is excessively high, theresultant golf ball tends to produce insufficient back-spin upon beinghit with an iron; i.e., controllability of the golf ball is lowered, andfeeling on impact is impaired.

The restitution elastic modulus of the cover material of the golf ballof the present invention is preferably at least 45%, more preferably 45to 85%, further preferably 50 to 80%, much more preferably 50 to 60%, asspecified by JIS-K7311. Since the thermoplastic polyurethane materialdoes not exhibit high restitution, preferably, the restitution elasticmodulus is strictly selected. When the restitution elastic modulus ofthe cover-forming material is excessively low, the total distance of thegolf ball is considerably lowered. In contrast, when the restitutionelastic modulus of the cover-forming material is excessively high, theinitial velocity of the golf ball becomes excessively high when beingshot or putted (i.e., when controllability of the golf is requiredwithin the range of a total distance of 100 yards or less), and the golfball may fail to meet a golfer's demand.

No particular limitation is imposed on the core employed in the golfball of the present invention, and any type of cores that are usuallyemployed can be employed. Examples of the core which may be employedinclude a solid core for a two-piece ball, a solid core having aplurality of vulcanized rubber layers, a solid core having a pluralityof resin layers, and a thread-wound core having a thread rubber layer.No particular limitation is imposed on the outer diameter, weight,hardness, and material of the core. The thickness of the golf ball coverof the present invention preferably falls within a range of 0.1 to 5.0mm. The cover may have a multi-layer structure, so long as the overallthickness of the cover falls within the above range.

The golf ball of the present invention is formed so as to have adiameter and a weight as specified under the Rules of Golf approved byR&A. Typically, the diameter is at least 42.67 mm, and the weight is45.93 g or less. The diameter is preferably 42.67 to 42.9 mm. Thedeformation amount of the golf ball under application of a load of 980 N(100 kg) is preferably 2.0 to 4.0 mm, more preferably 2.2 to 3.8 mm.

EXAMPLES

The present invention will next be described in detail by way ofExamples, which should not be construed as limiting the inventionthereto.

Examples and Comparative Examples

Core composition Polybutadiene rubber 100 parts by weight Zinc acrylate21.5 parts by weight Zinc oxide 12 parts by weight Dicumyl peroxide 1part by weight

The components of the aforementioned core composition were kneaded, andthen subjected to vulcanization and forming at 155° C. for 20 minutes,to thereby obtain a solid core for a two-piece solid golf ball(diameter: 38.5 mm). BR01 product of Japan Synthetic Rubber Co., Ltd.)was employed as the polybutadiene rubber. The specific gravity of thethus-obtained core was 1.07; the deformation amount under application ofa load of 980 N (100 kg) was 3.4 mm; and the initial velocity asmeasured by means of a method specified by USGA (R&A) was 78.1 m/s.

Cover materials shown in Tables 1 and 2 (unit: part(s) by weight) werekneaded by use of a twin-screw extruder at 190° C., to thereby obtaincover-forming materials. Components shown in Tables 1 and 2 aredescribed below.

Polyurethane 1 (Thermoplastic Polyurethane Material)

Pandex T8290: MDI-PTMG-type thermoplastic polyurethane material (productof DIC Bayer Polymer Ltd.) (JIS A surface hardness: 93, restitutionelastic modulus: 52%)

Polyurethane 2 (Thermoplastic Polyurethane Material)

Pandex T8295: MDI-PTMG-type thermoplastic polyurethane material (productof DIC Bayer Polymer Ltd.) (JIS A surface hardness: 97, restitutionelastic modulus: 44%)

Polyurethane 3 (Thermoplastic Polyurethane Material)

Pandex T8260: MDI-PTMG-type thermoplastic polyurethane material (productof DIC Bayer Polymer Ltd.) (Surface hardness as measured by use of aD-type durometer: 56, restitution elastic modulus: 45%)

Polyurethane 4 (Thermoplastic Polyurethane Material)

Pandex T7298: Non-yellowing-type thermoplastic polyurethane materialcontaining aliphatic isocyanate (product of DIC Bayer Polymer Ltd.) (JISA surface hardness: 98, restitution elastic modulus: 54%)

Hydrogenated MDI

Desmodule W: Dicyclohexylmethane-4,4′-diisocyanate (hydrogenatedMDI)(product of ACI Japan Ltd.)

Isocyanate 1 (Isocyanate Mixture)

Crossnate EM30: Isocyanate master batch (product of Dainichiseika Color& Chemicals Mfg. Co., Ltd.) containing 30% 4,4′-diphenylmethanediisocyanate (isocyanate concentration as measured through amine backtitration according to JIS-K1556: 5-10%, master batch base resin:polyester elastomer)

Isocyanate 2 (Isocyanate Compound)

Desmodur TT: Tolylene diisocyanate (TDI) dimer (product of SumitomoBayer Co., Ltd.) (effective NCO content: 24 to 24.6 weight %,isocyanate: TDI)

Subsequently, each of the aforementioned solid cores was placed in amold for injection molding, and a cover (thickness: 2.1 mm) was formedfrom each of the cover materials—obtained by dry-mixing the components(A) and (B))—around the core by means of injection molding, to therebyproduce a two-piece solid golf ball (Examples and Comparative Examples).The resultant golf ball was allowed to stand at room temperature for oneweek, and then properties of the golf ball were evaluated. Theevaluation methods are described below. A sheet (thickness: 2 mm) formedthrough injection molding was allowed to stand at room temperature forone week, and then subjected to measurement of cover properties.Furthermore, recyclability (i.e., formability) of the cover material wasevaluated. The results are shown in Tables 1 and 2.

Cover Properties

Surface Hardness

The surface hardness of the cover was measured by use of a D-typedurometer in accordance with JIS-K6253.

Restitution Elastic Modulus

The restitution elastic modulus of the cover was measured in accordancewith JIS-K7311.

Ball Properties

Hardness

The deformation amount of the golf ball under application of a load of980 N (100 kg) was measured.

Initial Velocity

The initial velocity of the golf ball was measured by means of a methodspecified by USGA (R&A).

Total Distance

The golf ball was hit at a head speed of 45 m/s by use of No. 1 wood (adriver) mounted on a swing robot machine, to thereby measure a totaldistance.

Scuff Resistance upon being Hit with an Iron

The golf ball was maintained at 23° C., 13° C., or 0° C., and then hitat a head speed of 33 m/s by use of a pitching wedge mounted on a swingrobot machine. Thereafter, the scuff resistance of the resultant golfball was visually evaluated on the basis of the following criteria.

5: No scuffing or substantially no scuffing is observed.

4: Scuffing is observed, but is negligible.

3: The surface of the ball is slightly scaly.

2: The surface of the ball is scaly, and a portion between dimples ofthe cover is lost to some extent.

1: A portion between dimples of the cover is completely exfoliated.

Yellowing (ΔYI)

A golf ball discoloration promoting test apparatus FM-1 (product of SugaTest Instruments Co., Ltd.) and a mercury lamp H400-F for color fadingtest (product of Toshiba) were employed. Each of the golf balls wasirradiated with light from the mercury lamp for 24 hours. ΔYI valuesbefore and after the irradiation treatment were measured through thereflection method specified in JIS-K7103 by use of a multi-light-sourcespectrophotometer MSC-IS-2DH (product of Suga Test Instruments Co.,Ltd.). The greater the value of ΔYI, the higher the degree of yellowing.

Formability

Recyclability of Cover Material

A runner resin generated during injection molding was pulverized andrecycled, and recyclability of the cover material was evaluated on thebasis of the following criteria. The term “runner resin” refers to aresin formed in a runner provided for uniformly feeding a molten resinto an injection molding machine. Typically, when a thermoplastic resinproduct is formed, a runner resin is pulverized and recycled by mixingwith a virgin resin.

Possible: When a pulverized runner resin (up to 50%) was mixed with avirgin resin, and the resultant mixture was formed into a product (i.e.,a golf ball cover), problems such as offset of a core did not arise.

Impossible: Since gelation of a runner resin occurred, and the runnerresin was not melted under application of heat, the resin could not berecycled.

TABLE 1 Cover material Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 (C) (A) (a-1)Polyurethane 1 50 Polyurethane 2 50 100 100 100 Polyurethane 3 100 (a-2)Hydrogenated MDI 1.5 1.5 3 1.5 1.5 Titanium oxide 3 3 3 3 3 Polyethylenewax 1 1 1 1 1 (B) Isocyanate 1 10 10 10 15 10 Cover Surface hardness 4649 49 52 58 properties Restitution elastic modulus (%) 50 46 46 48 48Ball properties Outer diameter (mm) 42.7 42.7 42.7 42.7 42.7 Weight (g)45.2 45.2 45.2 45.3 45.3 Hardness (mm) 3.2 3.1 3.1 2.9 2.6 Initialvelocity (m/s) 77.1 77 77 77.2 77.3 Total distance (m) 226 227 227 227228 Scuff resistance at 23° C. 5 5 5 5 5 at 13° C. 5 5 5 5 5 at 0° C. 55 5 5 4 ΔYI 3.5 3.5 3.5 3.5 3.5 Formability Recyclability PossiblePossible Possible Possible Possible

TABLE 2 Comp. Comp. Comp. Comp. Comp. Cover material Ex. 1 Ex. 2 Ex. 3Ex. 4 Ex. 5 (C) (A) (a-1) Polyurethane 1 50 Polyurethane 2 50 100 100Polyurethane 3 100 Polyurethane 4 100 Titanium oxide 3 3 3 3 3Polyethylene wax 1 1 1 1 1 (B) Isocyanate 1 20 20 20 Isocyanate 2 1.5Cover Surface hardness 47 53 58 47 51 properties Restitution elasticmodulus (%) 50 48 48 45 48 Ball properties Outer diameter (mm) 42.7 42.742.7 42.7 42.7 Weight (g) 45.2 45.3 45.3 45.1 45.2 Hardness (mm) 3.3 2.82.6 3.2 3 Initial velocity (m/s) 77.1 77.2 77.3 76.7 77 Total distance(m) 226 227 228 224 226 Scuff resistance at 23° C. 5 5 5 3 4 at 13° C. 55 5 2 3 at 0° C. 4 4 4 1 2 ΔYI 6.5 6.5 6.5 3 5 Formability RecyclabilityPossible Possible Possible Possible Impossible

As is clear from Tables 1 and 2, the golf balls of the Examples exhibithigh restitution and excellent flight performance. The results show thatthe golf balls of the Examples exhibit excellent scuff resistance uponbeing hit with an iron and excellent discoloration resistance. Incontrast, each of the golf balls of the Comparative Examples—in whichthe cover was not produced from the cover material of the presentinvention—exhibits poor restitution, and is not satisfactory in terms ofscuff resistance upon being hit with an iron and of discolorationresistance. The golf ball of Comparative Example 5 exhibits relativelygood ball properties. However, since the reactivity of the covermaterial after forming is excessively high, and gelation of the materialoccurs; i.e., the material is no longer melted under application ofheat, a runner resin generated during forming cannot be recycled.

As described above, according to the present invention, there can beproduced a golf ball having a cover which can be recycled for moldingand exhibits high restitution and excellent scuff resistance anddiscoloration resistance.

What is claimed is:
 1. A golf ball comprising a core and a cover therefor, wherein the cover is formed from a composition (C) containing, as predominant components, the following components (A) and (B): (A) a polyurethane material mixture of a thermoplastic polyurethane material (a-1) and dicyclohexyl-4,4′-methane diisocyanate (a-2), and (B) an isocyanate mixture in which an isocyanate compound (b-1) having at least two isocyanate groups serving as functional groups in the molecule is dispersed in a thermoplastic resin (b-2) which is substantially non-reactive with the isocyanate groups.
 2. A golf ball according to claim 1, wherein the thermoplastic polyurethane material (a-1) is synthesized from a polyether polyol and an aromatic diisocyanate.
 3. A golf ball according to claim 2, wherein the polyether polyol is a polytetramethylene glycol having an average molecular weight of at least 2,000.
 4. A golf ball according to claim 2, wherein the aromatic diisocyanate is 4,4′-diphenylmethane diisocyanate.
 5. A golf ball according to claim 1, wherein, in the polyurethane material mixture (A), the ratio by weight of the thermoplastic polyurethane material (a-1) to dicyclohexyl-4,4′-methane diisocyanate (a-2) is 100:0.5 to 100:5.
 6. A golf ball according to claim 1, wherein the isocyanate compound (b-1) in the isocyanate mixture (B) is 4,4′-diphenylmethane diisocyanate.
 7. A golf ball according to claim 1, wherein the thermoplastic resin (b-2) in the isocyanate mixture (B) is a thermoplastic polyester resin.
 8. A golf ball according to claim 1, wherein in the isocyanate mixture (B), the ratio by weight of the thermoplastic resin (b-2) to the isocyanate compound (b-1) is 100:5 to 100:100.
 9. A golf ball according to claim 1, wherein, in the composition (C), the ratio by weight of the thermoplastic polyurethane material (A) to the isocyanate mixture (B) is 100:1 to 100:40.
 10. A golf ball according to claim 1, wherein the material of the cover has a surface hardness of 40 to 80 as measured by use of a D-type durometer, and a restitution elastic modulus of at least 45%.
 11. A golf ball according to claim 1, wherein the material of the cover can be recycled for molding. 